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Fries Rearrangement

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Trehan et al reported a simple procedure for carrying out the Fries rearrangement of aryl esters to ortho- and para hydroxy acetophenones in dry open media in ordinary glassware using a commercial microwave oven.
 
The Fries rearrangement normally requires a Lewis acid and long reflux times or photochemical conditions. It has been observed that protected N-allyl anilnes with different substitution on the a ring.

When phenols are treated with acid chlorides or acid anhydrides, esters are obtained. Conversion of phenolic esters, on warming with a Lewis acid (Friedel-Crafts catalyst) to hydroxy ketones is known as Fries rearrangement.

Fries Rearrangement

Mechanism

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The reaction of aryl esters with Lewis acids to give ortho and para hydroxy ketones is known as Fries rearrangement. The Fries Rearrangement of Phenyl Acetate is given below.

Fries Rearrangement of Phenyl Acetate

Several theories have been proposed for the Fries rearrangement, but without any concrete result. There is evidence for the intermolecular as well as intramolecular mechanism and hence the rearrangement is best regarded as a combination of two mechanisms occurring simultaneously.
The presence of meta-directing group on the ring interferes with the reaction as in Friedel-Crafts reaction. Fries rearrangement is a reaction of great synthetic importance and has provided intermediates for many organic compounds. It appears to be typical acylation reaction in which the ester acts as a source of acyl carbocation. 

Fries Rearrangement Mechanism 

It has been found that the quantum yield of the photo Fries rearrangement product from p-tolyl acetate is independent of solvent viscosity, suggesting a tight transition state for the rearrangement. The mechanism of photo-Fries rearrangement is as follows. 

Photo Fries Rearrangement

Procedure

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For Fries rearrangement the mechanism has not been completely worked out. Opinions have been expressed that it is completely intermolecular, others believe it is completely intramolecular while still other believe that it is partially intramolecular and partially intermolecular.
To decide between the two processes, is to run the reaction of phenolic ester in presence of another aromatic compound say toluene. If some toluene is acylated the reaction should be at least in part intermolecular. If toluene is not acylated the reaction is intramolecular.
The fries rearrangement reaction has been shown by the fact that a mixture of products obtained when aluminium chloride is added to a solution of o-chloro p-tolyl acetate and p-tolyl benzoate.

Fries Rearrangement Procedure

A number of such crossover experiments have been carried out and sometimes crossover products have been found and sometimes not.

Acetanilide

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It is well known that the N-acylanilines when treated with polyphosphoric as well as with Lewis acids afford ortho and para acylanilines and N,N-Diacylanilines also rearrange upon heating in the presence of ZnCl2 to give para-acylanilines. 

Fries Rearrangement Acetanilide

However, this acid catalyzed Fries-like rearrangement leads to very low yields of the target products and therefore found no serious application for organic synthesis.